The present invention relates to medical sensors which include coded calibration information relating to characteristics of the sensor.
An example of such an encoding mechanism is shown in U.S. Pat. No. 4,700,708. This relates to an optical oximeter probe which uses a pair of light emitting diodes (LEDs) to shine light through a finger, with a detector picking up light which has not been absorbed by oxygen in the blood. The operation depends upon knowing the wavelength of the LEDs. Since the wavelength of LEDs actually manufactured can vary, a resistor is placed in the sensor with the value of the resistor corresponding to the actual wavelength of at least one of the LEDs. When the instrument is turned on, it first applies a current to the coding resistor and measures the voltage to determine the value of the resistor and thus the value of the wavelength of the LED in the probe. A disadvantage of this system is that it is dependent upon an analog amplitude level which can be affected by wiring impedance, noise, etc. Another disadvantage is that considerations of cost and error budget limit the number of distinguishable resistance values that may be employed, so that the amount of information conveyable by this means is limited.
Another method of storing coded information regarding the characteristics of the LEDs is shown in U.S. Pat. No. 4,942,877. This patent discloses using an EPROM memory to store digital information, which can be provided in parallel or serially from the sensor probe to the remote oximeter. This system either requires more wires to read the data in parallel, or requires the reading of a multiple, serial bit pattern rather than a single amplitude level as in the system using the resistor. In addition, an oximeter designed to read a resistor would be incompatible with a probe having such an EPROM memory structure.
Other examples of coding sensor characteristics exist in other areas. In U.S. Pat. No. 4,446,715, assigned to Camino Laboratories, Inc., a number of resistors are used to provide coded information regarding the characteristics of a pressure transducer. U.S. Pat. No. 3,790,910 discloses another pressure transducer with a ROM storing characteristics of the individual transducer. U.S. Pat. No. 4,303,984 shows another sensor with digital characterization information stored in a PROM, which is read serially using a shift register.